1. Field of the Invention
This invention relates to an image display system and, more particularly, to an image display system using a display medium having a fixed display screen resolution, e.g., a liquid crystal display panel or a plasma display panel.
2. Description of the Prior Art
Technical revolution has recently been advanced in the field of display panel such as liquid crystal and plasma display panels as well, and small-size light-weight computers utilizing such display panels are getting popular.
FIG. 3 shows a prior art image display system in a personal computer of the type noted above. In the FIGURE, reference numeral 1 designates an input source circuit, which consists of a microprocessor or the like and supplies pixel data to a refresh memory 8. Reference numeral 2 designates a clock source circuit including a plurality of clock sources CL1, CL2, . . ., CLN. N clock outputs from the clock source circuit 2 are supplied to a clock selection circuit 3. One of these clock outputs of the clock sources CL1 to CLN is selected by an instruction from the input source circuit 1 to be supplied to a sync signal generator 4 and a display timing signal generator 5. Reference numeral 6 designates a memory address generator, which receives signals from the sync and display timing signal generators 4 and 5 and provides an address of the refresh memory 8, in which pixel data for display is stored.
Reference numeral 7 designates an address selector, which selects an address of the input source circuit 1 when the input source circuit 1 reads or writes pixel data of the refresh memory 8 and selects an output address from the memory address generator 6 when pixel data in the refresh memory 8 is read out for display. The pixel data read out from the refresh memory 8 according to the output address from the memory address generator 6 is supplied to a display data processor 9 for conversion to a signal conforming to a form of input to a display panel (i.e., display medium) 10 in synchronism to sync signals.
Currently, there are a variety of application software including those developed in the past to be run on personal computers, while the resolution of the CRT display screen is on an increasing trend. Consequently, it has become necessary for a personal computer to be used with application software having display screens of various resolutions. For this reason, N different clock sources are provided as in the clock source circuit 2 shown in FIG. 3, and an adequate clock frequency conforming to the display screen resolution required by the application software used is selected according to an instruction provided from the input source circuit 1 to the clock selection circuit 3.
In a usual personal computer, a program, with which a microprocessor in the input source circuit 1 controls the circuit of FIG. 3, is peculier to each personal computer and conforms to the pertinent circuit construction. It is called BIOS (basic input/output system). Application software usually controls the circuit of FIG. 3 not directly but indirectly by calling an adequate program in the BIOS. In other words, the application software selects a clock source suited to its display screen resolution through a program in the BIOS. However, some application software is adapted to control the circuit of FIG. 3 directly, i.e., without agency of any BIOS.
Meanwhile, in the system of FIG. 3 using a display panel as display medium, the resolution of the display panel is fixed, for instance to 640 dots by 480 dots. This means that a clock source corresponding to the 640-dot-by-480-dot resolution should be selected at all time even if the display screen resolution of application software which is used with the system is 320 dots by 200 dots or 640 dots by 350 dots.
Therefore, if application software which directly controls the clock source switching is used with a personal computer having an image display section as shown in FIG. 3, for instance consisting of a liquid crystal display panel having a resolution of 640 dots by 480 dots, it will switch the clock source circuit 3 in accordance wiht its own 640-dot-by-350 -dot resolution display screen. In this case, normal image display on the display panel will not be obtained, or the display panel will be destroyed.